1. Field
The present invention relates to a light emitting device and a method for fabricating the same.
2. Description of the Related Art
Generally, sapphire (Al2O3) used for a substrate for growing gallium nitride (GaN) causes many problems in fabrication and driving of devices due to its own nonconductivity and low thermal conductivity in fabricating the devices using GaN.
To solve such problems, a device is fabricated by removing a sapphire substrate using an LLO (Laser Lift Off) method. In order to remove the sapphire substrate, a GaN thin film should be first bonded to a wafer made of silicone (Si), or gallium arsenide (GaAs), or a metal plate, which has high conductivity and superior thermal conductivity.
As described above, if a wafer or a metal plate is bonded to a GaAs thin film, chances are that a portion of the GaAs thin film is broken or several cracks are generated therein.
As one of the methods for reducing such break of the GaN thin film and propagation of the cracks, etching a portion of the GaN thin film and bonding it to the surface of sapphire has been widely used.
However, due to a step naturally produced after the etching, a void space is produced upon bonding of the GaN film. Accordingly, a process of filling such a void space with a variety of materials and subsequently performing a bonding process has been frequently used.
FIGS. 1a to 1e are sectional views schematically illustrating a conventional process of fabricating a GaN device. As shown in FIG. 1a, a thin film layer (11) including a plurality of devices each of which has an N-GaN layer, an active layer and a P—GaN layer is formed on a top surface of a sapphire substrate (10).
Here, the device is a device such as a light emitting diode.
Thereafter, the thin film layer (11) is selectively etched such that the plurality of devices (11a) are separated from each other (FIG. 1b).
At this time, due to the etching process, regions that have been etched between the devices (11a) become trenches (20) defined and enclosed by the devices (11a).
Next, a P metal layer (12) is formed on the top of each of the plurality of devices (11a) (FIG. 1c).
Successively, a carrier substrate (14) is bonded to the P metal layers (12) formed on each top of the plurality of devices (11a) using a bonding material (13) (FIG. 1d).
The carrier substrate (14) is made of one selected from the group consisting of silicone, GaAs, Cu and Al.
Then, the sapphire substrate (10) is separated by performing an LLO process (FIG. 1e).
Here, there arises a problem in that the trenches (20) are not filled with the bonding material (13) so that, as shown in FIG. 1e, a crack (25) is produced in the device due to the difference of a thermal expansion coefficient between GaN and air expanded by heat of laser incident for the LLO process.
FIGS. 2a to 2h are sectional views schematically illustrating another conventional process of fabricating a GaN device. A thin film layer (11) including a plurality of devices each having an N-GaN layer, an active layer and a P—GaN layer is formed on a top surface of a sapphire substrate (10) (FIG. 2a).
Here, the device is a device such as a light emitting diode.
Thereafter, the thin film layer (11) is selectively etched such that the plurality of devices (11a) are separated from each other, thereby making trenches (20) between the devices (11a) through the etching (FIG. 2b).
Next, a P metal layer (12) is formed on the top of each of the plurality of devices (11a), and the trenches (20) are filed with an easily removable material (30) (FIG. 2c).
Here, the easily removable material (30) is any one of epoxy, a photoresist, a polyimide and a dielectric.
Successively, a metallic bonding layer (33) is deposited on the P metal layer (12) formed on the top of each of the plurality of devices (11a), and another metallic bonding layer (34) is deposited on the bottom of a carrier substrate (35). Then, the P metal layers (12) and the carrier substrate (35) are bonded by means of bonding forces of the metallic bonding layers (33, 34) (FIG. 2d).
Here, a material such as AuSn with a melting point of about 350° C. is used for the metallic bonding layers (33, 34). The carrier substrate (35) is put on the top surfaces of the P metal layers (12), and the metallic bonding layers (33, 34) are then melted at a temperature higher than the melting point of the metallic bonding layers (33, 34) so that the P metal layers (12) and the carrier substrate (35) are bonded to each other.
Then, the sapphire substrate (10) is separated by performing an LLO process, and the material (30) filled in the trenches (20) is removed (FIG. 2e).
Thereafter, the plurality of devices (11a) and the trenches (20) are cleaned, and the plurality of devices (11a) are partially removed by performing an etching process (FIG. 2f).
Here, the removed regions of the plurality of devices (11a) are regions of the devices opposite to regions where the P metal layers (12) are formed.
Except for the portions of the top surfaces of the plurality of devices (11a) which have been partially removed, a passivation film (18) for enclosing the plurality of devices (11a) and filling the trenches (20) is formed, and N metal layers (15) are then formed on the portions of the top surfaces of the plurality of devices (11a) where the passivation film (18) has not been formed (FIG. 2g).
Subsequently, the plurality of devices (11a) are separated to pieces from each other by performing a scribing process and a breaking process (FIG. 2h).
FIGS. 3a to 3h are sectional views schematically illustrating a further conventional process of fabricating a GaN device. A thin film layer (11) including a plurality of devices each having an N-GaN layer, an active layer and a P—GaN layer is formed on the top surface of a sapphire substrate (10) (FIG. 3a).
Thereafter, the thin film layer (11) is selectively etched such that the plurality of devices (11a) are separated from each other, thereby forming trenches (20) between the devices (11a) through the etching (FIG. 3b).
Next, a P metal layer (12) is formed on the top of each of the plurality of devices (11a), and the trenches (20) are filled with an easily removable material (30) (FIG. 3c).
Here, the easily removable material (30) refers to such material as can be easily removed through an etching process.
The foregoing processes of FIGS. 3a to 3c are the same as those of FIGS. 2a to 2c. 
Subsequently, a seed metal layer (40) is deposited on the tops of the filling material (30) and the P metal layers (12) formed on the plurality of devices (11a), and a metallic carrier layer (41) is then deposited on the seed metal layer (40) (FIG. 3d).
Then, the sapphire substrate (10) is separated by performing an LLO process, and the material (30) filled in the trenches (20) is removed (FIG. 3e).
Thereafter, the plurality of devices (11a) and the trenches (20) are cleaned, and the plurality of devices (11a) are partially removed by performing an etching process (FIG. 3f).
Here, the removed regions of the plurality of devices (11a) are regions of the devices opposite to regions where the P metal layers (12) are formed.
Except for the portions of the top surfaces of the plurality of devices (11a) which have been partially removed as such, a passivation film (18) for enclosing the plurality of devices (11a) and filling the trenches (20) is formed, and N metal layers (15) are then formed on the portions of the top surfaces of the plurality of devices (11a) where the passivation film (18) has not been formed (FIG. 3g).
Successively, the plurality of devices (11a) are separated to pieces from each other by performing a scribing process and a breaking process (FIG. 3h).
The aforementioned second method of FIGS. 2a to 2h and third method of FIGS. 3a to 3h are quite similar to each other in view of all the processes. As compared with the aforementioned first method, the trenches in the second and third methods are filled with an easily removable material, and metal layers are bonded to each other to ensure strong bonding, remarkably reducing the frequency of occurrence of fine voids that may be produced during the bonding. However, the problem of there being presence of a crack persists.